What is sulfur dioxide?

Sulfur dioxide (SO2) is one of a group of highly reactive gases called sulfur oxides (SOX). Exposure to SO2 can affect the respiratory system, especially for people with asthma. Studies show connections between short-term exposure and increased visits to emergency departments and hospital admissions for respiratory illnesses, particularly in populations at risk (including children, the elderly, and asthmatics). SOX can react with other compounds in the atmosphere to form small particles that cause or worsen respiratory disease or aggravate existing heart disease.

The largest source of SO2 emissions is fossil fuel combustion at power plants and other industrial facilities. SO2 emissions also come from extraction of metal from ore and burning high-sulfur fuels in locomotives, large ships, and non-road equipment.

National Ambient Air Quality Standards (NAAQS) for SO2

History of the SO2 NAAQS

The United States Environmental Protection Agency (EPA) issued annual and 24-hour primary (health-based) SO2 standards as well as three-hour and annual secondary SO2 standards in 1971. As a result of the EPA's review of the SO2 NAAQS in 1973, the secondary annual SO2 standard was revoked and the secondary three-hour standard was retained. In 1996, following a subsequen review of the NAAQS, the EPA retained both the annual and 24-hour primary SO2 standards.

Effective August 23, 2010, the EPA strengthened the SO2 NAAQS with a new primary one-hour standard of 75 parts per billion (ppb) (75 FR 35520). A one-hour standard was determined to better protect the public from exposure to high short-term SO2 concentrations, especially in communities located close to coal-fired power plants, industrial boilers, petroleum refineries, metal processing plants, and diesel exhausts.

In setting the 2010 primary one-hour SO2 standard, the EPA revoked the annual and 24-hour primary standards for areas not previously designated nonattainment, to become effective one year after designations for the 2010 standard are finalized. For areas of the country previously designated nonattainment for SO2 under the 1971 standards, the annual and 24-hour standards remain in effect until state implementation plans to attain or maintain the 2010 standard are approved by the EPA.

Texas Compliance with the SO2 NAAQS

Texas currently has no SO2 nonattainment areas. No areas of Texas have yet been designated under the 2010 one-hour SO2 air quality standard.

In April 2012, Texas recommended to the EPA that Dallas, Ellis, El Paso, Galveston, Gregg, Harris, Jefferson, Kaufman, McLennan, and Nueces Counties be designated attainment for the 2010 SO2 standard; and all remaining counties be designated unclassifiable. Texas' recommendation was based on 2011 design values for the state's existing SO2 regulatory monitors.

On August 5, 2013, the EPA published final nonattainment area designations for areas with 2009 through 2011 monitoring data indicating violations of the one-hour SO2 standard (78 FR 47191) . No areas of Texas were designated.

Implementation of the 2010 Primary SO2 NAAQS

The EPA’s February 2013 strategy paper indicates designations for remaining areas of the country (including all areas of Texas) will be made in future actions.

In January 2014, the EPA issued updated monitoring and modeling technical assistance documents. The EPA provided SIP guidance for nonattainment areas on April 23, 2014. On May 13, 2014, the EPA proposed options, data requirements, and schedules for states to characterize and assess SO2 air quality near relevant emission sources (79 FR 27446). Finalization of the rule is anticipated in late 2014.

Latest Air Quality Planning Activities

Infrastructure and Transport SIP Revision for the 2010 SO2 NAAQS

On April 23, 2013, the commission adopted the Federal Clean Air Act (FCAA), §110(a)(1) and (2) Infrastructure and Transport SIP Revision for the 2010 SO2 NAAQS (Project No. 2012-022-SIP-NR).

The SIP revision identifies basic program elements enabling Texas to meet infrastructure requirements for the 2010 SO2 NAAQS, as stipulated in the FCAA. The revision specifically addresses FCAA, §110(a)(2)(D)(i)(I) transport requirements with a technical demonstration showing Texas does not contribute significantly to nonattainment nor interfere with maintenance of the SO2 NAAQS in any other state.